The discovery of the fluidized bed process for the production of polymers provided a means for producing a diversity of polymers, e.g., polyolefins such as polyethylene, with a drastic reduction in capital investment and a dramatic reduction in energy requirements as compared to then conventional processes. The present invention provides a means for even greater savings in energy and capital cost by affording a simple and efficient means for obtaining a substantial increase in production rate in a given size reactor over what was previously possible in a fluidized bed process.
A limiting factor in the rate of production of polymer in a fluidized bed is the rate at which heat can be removed. The most common and perhaps universal means of heat removal employed in conventional fluidized bed reactor processes is by compression and cooling of the recycle gas stream at a point external to the reactor. In commercial scale fluidized bed reaction systems for producing polymers such as polyethylene, the amount of fluid which must be circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate solids mixing in the fluidized bed. However, the fluid velocity in the reactor is limited to the extent necessary to prevent excessive entrainment of solids in the fluidizing gas as it exits the reactor. A constant bed temperature will result if the heat generated by the polymerization reaction (which is proportional to the polymer production rate) is equal to the heat absorbed by the fluidizing stream as it passes through the bed, plus any heat removed or lost by other means.
It has long been assumed that the recycle gas temperature could not be lowered any further than to a point slightly above the dew point of the recycle gas stream, the dew point being the temperature at which liquid condensate begins to form in the gas stream. This assumption was predicated on the belief that the introduction of liquid into a gas phase fluidized bed reactor would inevitably result in plugging of the recycle lines, heat exchanger or below the fluidized bed or gas distributor plate, (if one is employed); non-uniformity of monomer concentrations inside the fluidized bed; and accumulation of liquid at the bottom of the reactor which would interfere with continuous operation or result in complete reactor shut-down. It was widely believed that, in the extreme case, liquid present in the fluidized bed of a gas phase process, could so disrupt the fluidization process in the bed as to result in a collapse of the bed and sintering of the solid particles into a solid mass. The perceived need to operate above the dew point to avoid liquid in the recycle stream has substantially restricted the production rate in reactors of a given size or, in the alternative, required the use of a substantially oversized reactor in manufacturing polyolefins to achieve acceptable production rates. The problem is exacerbated in situations where a reactor has been sized for a homopolymer and it is desired to use the reactor to manufacture products, such as those using hexene as a comonomer, where the dew point of the recycle stream is relatively high. Production rates in such situations have been even more severely restricted by past practices. The care exercised in accordance with this widely held belief to avoid the presence of any liquid in the recycle stream when it enters the reaction zone (fluidized bed) is evident from the teachings of U.S Pat. Nos. 3,922,322 and 4,035,560 and European Patent Application' No. 0 050 477. U.S. Pat. No. 4,359,561 teaches that the temperature of the recycle stream at the outlet of the recycle heat exchange zone should be limited to a temperature at least about 3.degree. to 10.degree. C. above its dew point.
Similarly, European Patent Application No. 0,100,879 published Feb. 2, 1984 states, in the context of a gas phase polymerization process for incorporating 1-olefin comonomers into a polymer chain of ethylene units that 1-olefin comonomers "normally require a higher feed temperature to prevent their condensation."
We have discovered, contrary to these widely held beliefs, that in continuous fluidized bed polymerization processes, the recycle stream can be cooled below the dew point to partially condense a portion thereof and the resulting stream containing entrained liquid returned to the reactor without plugging or other problems which would interfere with continuous reactor operation. Such operation has been called "condensing mode" polymerization operation and is disclosed in prior U.S. application Ser. No. 361,547, filed Mar. 24, 1982 and U.S. application Ser. No. 594,962, filed Apr. 3, 1984. As there disclosed, the employment of the condensing mode permits a reduction in the recycle stream temperature to a point below the dew point which results in a marked increase in space-time-yield for the continuous polymerization process over that obtainable in the non-condensing mode of operation of the prior art.
The primary limitation on reaction rate in a fluidized bed reactor is the rate at which heat can be removed from the polymerization zone. Although they differ in very important ways from gas fluidized bed reaction systems, the same heat limitation problems exist in other types of reaction systems such as stirred, gas-phase reaction systems and, to some extent, slurry reaction systems.
In U.S. Pat. No. 3,256,263, heat removal in a stirred reaction system is achieved by the compression of recycle gases and expansion upon reentry into the reactor. In other stirred or paddle-type reaction systems some additional cooling is effected by the injection of liquid onto the top of the bed. See for example, U.S. Pat. Nos. 3,254,070, 3,300,457 and 3,652,527.
In U.S. Pat. Nos. 3,965,083, 3,970,611 and 3,971,768 assigned to Standard Oil Co. (Indiana), cooling of a stirred bed reactor is supplemented by injection of liquids on the top of the bed.
In U.S. Pat. No. 4,012,573 (Trieschmann et al) gases withdrawn from a stirred reactor are condensed to liquid and returned in liquid form to the stirred reactor where the liquid is brought into desired contact with polymer in the stirred bed.
Mitsubishi Petrochemical Co. has proposed the use of liquids or regasified liquids for cooling in a gas phase reactor (J55/045,744/80, U.S. Pat. No. 3,944,534 and DT 2 139 182). In these disclosures all of the liquid or regasified liquid is injected directly into the bed rather than entering with the fluidizing gas as in the present invention. DT 2 139 182 is specific to stirred beds rather than fluidized beds. In J55/045,744/80 the liquid is regasified before being injected into the fluidized bed.
In Japanese Patent Publication No. 4608/81, Mitsubishi Petrochemical Company has proposed the use of an inert hydrocarbon in liquid form in a process described as gaseous phase polymerization oF olefins. The hydrocarbon having three or more carbon atoms is allowed to be present in liquid form in an amount corresponding to 1-20 weight percent of the polymer granules in the vessel. The liquid hydrocarbon is described as useful for avoiding electrostatic adhesion of the polymer granules to the exterior parts of the polymerization vessel. Apparently, to achieve the desired results, the Publication contemplates that the liquid will not evaporate but rather will be present throughout the process in its liquid state. The only exemplary support in the Publication is a stirred vessel, apparently of lab scale size since a charge of 20 grams of polyethylene powder was used to initiate the process. However, according to the Publication, the vessel may be of any existing type For gaseous phase polymerizations, e.g., a fluidized bed type or an autoclave type equipped with an agitator.
In U.S. Pat. No. 3,625,932, assigned to Phillips Petroleum Co., a multiple-stage fluidized bed reactor for the polymerization of vinyl chloride is described. ln the reactor liquid monomer is introduced through distribution rings below each bed of polymer particles where it vaporizes immediately, thus absorbing large amounts of heat of reaction, and, together with gaseous monomer introduced into the bottom of the reactor vessel, serves to fluidize the beds of polymer particles.
In a fluidized bed reaction system, as distinguished from stirred or paddle-type reaction systems where agitation by mechanical means is available, uniform distribution of monomer and catalysts in the upwardly moving gas stream is essential to avoid hot spots and resulting polymer chunks. In stirred and paddle-type reactors these problems are overcome by mechanical stirring and agitation which serve to (1) attain contact between the two phases, i.e., the gas phase and the solid particles phase, (2) assist in providing uniform temperature throughout the reaction vessel and (3) reduce the tendency for polymer chunks to form or "sheeting" to occur, i.e., polymer buildup on surfaces of the reactor system. These potential problems must be dealt with in a fluidized bed system without the benefit of mechanical agitation. Additionally, the velocity of gas flowing through the reactor must be adequate to maintain the bed in a fluidized state. The gas velocity required to keep the bed in a fluidized suspension cannot be achieved under normal conditions by mere injection of liquid at the bottom of the bed. Therefore, the direct liquid injection cooling of a reactor, as described by Trieschmann et al, is not a viable option for a fluidized bed reaction system.